1. Field of the Invention
This invention relates to an internal grooved tube used as a heat exchanger tube for a heat exchanger of a refrigerator and an air conditioner or the like and a method of manufacturing such an internal grooved tube, and more specifically, to an internal grooved tube having a large number of fine spiral grooves (or fins) formed on the inside surface in parallel arrangement at a certain pitch and a method of manufacturing such an internal grooved tube.
2. Description of the Related Art
The promotion of miniaturization, higher performance and energy conservation has been made as to a heat exchanger. In this connection, as an internal grooved tube to meet such demands, in Japanese Patent Laid-open No. 8-21696, for instance, there is proposed a heat exchanger tube having spiral grooves of a great height on the inside surface and fins of a sharp vertical angle.
As a method of manufacturing an internal grooved tube, in Japanese Patent Laid-open No. 54-37059, there is disclosed a method of manufacturing a heat exchanger tube by the steps of inserting a grooved plug having a large number of fine spiral grooves on the outside surface into a blank tube rotatably, then pressing the blank tube against the outside surface of the grooved plug with a plurality of rolls arranged to revolve both around the circumference of the blank tube and on its axis in a location of the grooved plug inserted, while drawing out the blank tube in one direction, and then using a holder to hold the roll axis for stabilizing the rotation of the rolls.
As a method for high-speed machining of an internal grooved tube, in Japanese Patent Laid-open No. 55-103215, there is disclosed a method of manufacturing a heat exchanger tube by the steps of inserting the grooved plug as described the above into a blank tube rotatably, and then pressing the blank tube against the outside surface of the grooved plug with balls densely arranged to revolve both around the circumference of the blank tube and on its axis in a location of the grooved plug inserted, while drawing out the blank tube in one direction.
The internal grooved tube disclosed in Japanese Patent Laid-open No. 8-21696 meets the requirements of spiral grooves of a great height and fins of a sharp vertical angle, permitting the achievement of the intended objects. However, with greater groove height (fin height), it is necessary to increase a thickness of a tube in proportion to the groove height, resulting in an increase in tube weight. Besides, large crushes of fins formed in the tube occur in tube expansion (by press-fitting a rod provided with a net ball at the tip for tube expansion to fix the tube to aluminum fins) for incorporating the tube into the heat exchanger, and as a result, the grooves formed to be of a great height could not often take satisfactory effect.
Among the internal grooved tube manufacturing methods, the method of permitting the planetary revolution of a plurality of rolls having axes held by the holder around the circumference of the blank tube in a location of the grooved plug inserted as disclosed in Japanese Patent Laid-open No. 54-37059 described the above requires a lubricating mechanism between the roll and the roll axis, in addition to the holder, for revolution of the rolls at high speed to increase a machining speed, resulting in an increase in roll diameter and also a complication of structure. For that reasons, an increase in number of revolutions of the rolls hinders the stability of the revolution of the roll and its rotation axis, and therefore, it is not possible to hold a stable orbit of revolution, resulting in a difficulty in increasing a grooving (rolling) speed.
In order to solve the above problems, the technique of arranging the balls densely, instead of the rolls, around the grooved plug location of the blank tube to be drawn out is developed, as disclosed in Japanese Patent Laid-open No. 55-103215 described the above. When the balls are in use in this manner, the balls and the blank tube make point-contact each other, permitting stable and higher-speed machining. Then, with an increase in number of balls, the balls might normally revolve around the circumference of the blank tube in a shorter period in the state of being pressed against the circumference of the blank tube to form the grooves on the inside surface of the tube by rolling, permitting more improved grooving workability, together with higher machining speed.
However, when the grooves of the grooved plug have a large lead angle to the axis, breakage (tear-off) of the blank tube occurs in process of machining to hinder higher-speed machining in spite of adding more balls. Thus, there has been a limit to manufacture of a high-performance heat exchanger tube having a large lead angle to the tube axis.